The present invention is an optical system having a lens array that includes plural, two-dimensionally arranged lens cells, and is used, for example, for an optical system used to illuminate the image modulation element or image forming element using a light from a light source.
A conventional projector or image projecting apparatus modulates a light from a light source by using an image modulation element, such as a liquid crystal panel, in accordance with an image signal, and projects the modulated light onto a screen via a projection lens. It is important for this projector to maintain uniform brightness and color and a high contrast on an image projected on a screen.
FIGS. 11A and 11B show an illustrative illumination optical system that illuminates an image modulation element in the projector. FIGS. 11A and 11B show an YZ section and an XZ section in the conventional illumination optical system. The Z-axis is an axis that extends in a central optical axis direction of the illumination optical system.
In these figures, the light exited from a light source 501 is collimated by a paraboloid reflector 502, and then split into plural lights by a first fly-eye lens 503. Each light is condensed near a second fly-eye lens 504, forming a (secondary) light source image. Plural lens cells in the fly-eye lenses 503 and 504 have a rectangular lens plane similar to a liquid crystal panel as an illuminated plane.
The polarization conversion element 505 aligns polarization directions of the split lights exited from the second fly-eye lens 504. The lights are then condensed by the condenser lens 506, and superimposed on the liquid crystal panel 508 through a color separating/synthesizing system (not shown) that provides a color separation and a color synthesis.
In general, an attempt to improve the light use efficiency in this illumination optical system is likely to increase a light angular distribution. Therefore, use of an optical element that has a characteristic or incident angle dependency sensitive to an angular distribution of the incident light in the illumination optical system would pose problems of the uneven brightness and color, the lowed contrast, and the deteriorated image quality. Illustrative optical element having the incident angle dependency are a mirror and a prism having a dichroic film and a polarization splitting film that are arranged oblique to the optical axis, and these optical elements are used for a color separation and a color synthesis.
As a solution for the problem of the image quality deterioration, Japanese Patent Application, Publication No. 11-295658 proposes a technology that decenters each lens cell in one of orthogonal lens cell arrangement directions, and narrows a light width incident upon the second lens array, thereby reducing an angular distribution in a direction sensitive to the light angular distribution.
Japanese Patent Application, Publication No. 2000-206463 proposes a technology that uses an approximately circular first lens array, and condenses the light to an approximately rectangular second lens array, thereby receiving the light from the light source as much as possible.
However, a lens cell in each lens array proposed in Japanese Patent Application, Publication No. 11-295658 is a spherical lens, and cannot provide decentering greater than a radius of curvature. In addition, when each lens cell 601a in a first lens array 601 is given large decentering as shown in FIG. 12, a light source image formed near a second lens array 602 destroys due to the (spherical) aberration that occurs in the lens cell 601a, lowering the illumination efficiency. Moreover, it is difficult to mold a significantly decentering lens cell, and the practicability is low.
A much narrower arrangement area width of the lens cell in the second lens array than that of the first lens array is needed to provide a practicable effect of image quality maintenance, but prior art is silent about such solutions.